Let's go through the design and testing of a RF choke, and see why and how
they work. Of course a choke can be tested using some sort of impedance meter,
such as a network analyzer or high quality impedance meter, but let's look at how to get by with minimal test gear.

Impedance Limits

The minimum required plate choke impedance varies with the operating impedance
of the output device, the choke's Q or loss resistances, and compensating capacitance
available in the tank tuning capacitor. The RF plate choke must have several
thousands of ohms when the amplifier uses high voltage tubes. RF plate choke
impedance must be high enough to limit RF choke current to safe values. A
conservative RF current estimate would be E/Z=I where E is DC plate voltage and
Z is choke impedance. A 200 uH choke on 1.8 MHz only has about 2600 ohms
reactance, so a 3000 volt anode swing causes 1.15 amperes peak RF current. RF
choke dissipation would be choke Erms^2 / Rp. A 200uH choke with a Q of 40 would
dissipate something less than 2100^2 / 104000 = 42 watts.

42 watts is significant power, although perfectly acceptable in a single
layer ceramic form choke of fairly large dimension. The very high Q
requirements and the large RF currents illustrate why iron core or ferrite core
chokes are generally unacceptable at high anode voltages, and why the general
trend is to use physically large solenoid chokes on low loss high temperature
forms.

Series resonances produce very low choke impedance. This is accompanied by
very high RF voltages from the choke to the choke ends and to the outside world.
I've seen chokes arc across several inches of air in amplifiers operating at 6-8
kV supply voltage.

Series resonances are formed when the choke looks like back-to-back L-networks.
The choke winding forms a long series inductance and stray capacitance tunes the
winding. This of course could be viewed as a long helical transmission line of
very high impedance, but L networks provide a closer if not significantly less
complex analogy. Electrically, at the lowest frequency series-resonant point, the choke looks like this:

The sweep waveform below shows voltage at point A.

This model very approximates an Ameritron plate choke. This is why 26-27 MHz
operation generally destroys an Ameritron choke.

This is with 200 volts peak-to-peak excitation, NOT the full voltage of the
amplifier. The waveform below shows RF current through the choke. DC current is
not shown, and would be superimposed on the RF current. 10 volts=1 ampere:

Ever wonder why an amplifier can arc and bang when operated near self-resonance
of the plate choke? The text above should explain it to you! Voltage at the
center of the choke can be so high the choke arcs for several inches. The choke
becomes a good Tesla coil, with peak RF voltage near the coil's middle for the
first-order resonance.

Mounting the Choke

At upper frequency limits, a typical choke can have peak voltages several times
the operating dc voltage at some points along the windings. Stray capacitances
also tend to concentrate RF currents into small areas of the winding. Because
stray capacitance aggravates voltage and current stresses, and because stray
capacitance shifts series-resonances lower in frequency, a clear location for
the choke is most desirable. If possible, a choke should be 1/2 its winding
length, or four times the winding diameter, away from things that add
capacitance. This includes large dielectrics, which increase stray capacitances
from increased dielectric factor loading of electric fields.

How to Move the Series Resonance

Choke designs require a
certain minimum inductance, to ensure reasonable impedance near lower frequency
limits. If the choke is physically large, and if reactance is fairly large at
the lowest frequency, and if a wide frequency range is covered, unwanted
series-resonances can fall within desired upper operating ranges. This can
result in very high currents and voltages from normal fundamental RF excitation,
although it is sometimes blamed on a "parasitic" by less knowledgeable designers
or technicians. The solution is to move undesired series-resonances outside
desired frequency ranges.

Many publications, including the ARRL Handbook, and many personal opinions,
claim
choke winding gap designs are "magic". At one time, the ARRL Handbook reported
there was no rational, logical, reasoning
behind removing sections of solenoid choke windings. This is not true, and
really only shows
those who make such statements do not understand why a choke has series
resonances, or how to move the resonances. If we understand why a choke
misbehaves on some frequencies, we will easily understand how to "correct"
problems in the RF choke by using gapped windings.
There actually is a method to designing choke winding gaps. Since the series-resonance
problem is rooted in the choke behaving like two (or multiples of two)
back-to-back L-networks, the solution is very obvious.

When moving unwanted series-resonances, the system's necessary minimum
inductance often rules out significant inductance reductions. Turns must be
added or removed where they have the largest effect on series resonances, with
minimal reduction of lower frequency inductance. To do this, the designer must find the highest voltage
area of the winding at the problematic frequency, and reduce capacitance
at that physical point in the winding.

The
capacitance involved in the series resonance is generally on the order of a few picofarads,
or less. The designer generally does not want to locate the choke center near metal (or even
dielectrics other than air) because metallic masses, or even dielectrics other
than air, will move series resonances lower in frequency.

Generally, the designer wants to do everything possible to move series resonances upwards
in frequency, keeping as many high-order resonances as possible above the
highest operating frequency.

The most expedient way to move series resonances is to change the winding pitch
at the very center of the choke for lowest resonance, or in the electric field's "hottest"
winding areas, in the case of
higher-order resonances. A typical solenoid choke of constant pitch has
capacitance controlling the lowest resonance located in the very middle of the winding
length. This is area where inductances, forming the two phantom back-to-back
L-networks, are evenly distributed in both directions. At this inductive center-point, just one or two picofarads of capacitance can move the
series resonance as much as 50% in frequency! The exact amount of movement
depends on winding pitch, form diameter, and the overall inductance of the
choke.

The gaps in the Ameritron chokes (or any gapped choke I design) are not placed by accident.
The winding gaps are placed by design.

The most effective way to move a resonance is to remove wire from the area where
voltage is at maximum.

A full winding choke is tested through all desired frequency ranges

If a series resonance appears inside a band, the highest voltage area is
located

Wire is removed from the highest voltage area to shift unwanted
resonance up

The choke is retested

The 1990's Ameritron choke design appears on the far left. Starting with a
continuous winding core, the fully wound choke had resonances at 10 MHz and 20 MHz. Looking at
voltage hot spots, sections of windings were removed. The lower gap near the
choke middle moved the 10 MHz resonance
up to 12.5 MHz or so. This shifted the upper resonance from 20 MHz to near the
24.8 MHz band. The upper gap moved the resulting second-order series-resonance from
25 MHz up to 27
MHz. Without the upper gap, the second overtone resonance
is too close to 24.8 MHz. The gaps park unwanted series-resonances between 30 and 20 meters, and at the
lower end of 11 meters. This results in the highest possible inductance for 160
meters, while keeping harmful resonances away from normal operating frequencies.

On early solenoid chokes, before we had WARC bands, resonances
were more easily parked in clear spots. There was no need to move higher
order resonances out of an "overtone" or harmonic relationship with the
lowest frequency
series resonance. The double gaps reflect a change in choke design from the
Heathkit and Ameritron chokes I designed before WARC bands existed.

Single section space-wound. 117 µH
nominal with resonances at 19.5 and 27.3 MHz.

Single section space-wound iron core.906 µH nominal with resonances at 13.5 and 24.1 MHz.
The iron core choke has low Q, and runs much hotter for a given RF current.
Series resonances are also very wide, the lower resonance rendering this choke
unusable between 13 and 14.5 MHz.

From the above data, we can reasonably conclude an evenly-spaced winding
produces the poorest multiple-band high-voltage choke. The choke can have a great deal more inductance by using
a tight
winding pitch, with harmful resonances moved by intentionally inserting large gaps
at appropriate places.

Testing the Choke

A standard transmitter, dummy load, and 12 volt lamp can be used to effectively
test an RF plate choke.

Electrically, keep the TL from the "T" point to the choke and lamp very short.
Less than five electrical degrees (one-half foot at upper HF) is generally short
enough.

The other two TL lengths are not critical.

The best place to test the choke is in the actual operating location, with the
choke cold end bypassed to ground normally, but the top end disconnected from
the tank system and tubes. The lamp would go between the disconnected top choke
end, and the TL supplying RF. The lead above the lamp to the TL can be somewhat
longer, even 5 inches is unlikely to have a large effect. The lead from the lamp
to the choke must be short.

TL from the tap point on the transmission line must be short, as in the bench
setup below!

4-section choke under test for series resonance. The transmitter is set at 25
watts and the VFO swept up through the frequency range until the lamp glows.
Adjust power so the lamp lights, but does not burn out. In this case series
lower resonance was at 12.985 MHz.

The width of frequency range for the glow roughly indicates Q based on
selectivity, while brightness at
a given power also indicates Q based on Rp. A brighter glow generally means higher Q, and
that's a very good thing.

Spinning the VFO up, we find the upper resonance around 16.8 MHz.

Finding the problem area:

By sliding a well-insulated metal tipped tool along the choke, the "hot spot" or
"hot spots" can be located.

Adjust the radio's frequency to find maximum lamp brightness. Without adjusting
the radio, move the insulated tool's metal tip along the choke and watch for the
spot where the bulb dims the most. This is the "hot spot" where voltage peaks.

If you remove wire in this hot area, the series resonance will shift upward the
maximum possible amount for a minimal effect on overall inductance. To lower
self resonant frequency, either add dielectric (a thick coating of insulating
varnish) or rewind with closer turns spacing.

The higher frequency resonances will be in two or more places, and are out near
the ends of the choke. The lowest frequency resonance is near the choke center.

When you find a frequency with the largest hand effect near the center of the
choke, you can be pretty sure you have the lowest self-resonant frequency.

Minimum required Inductance and Choke Current

The minimum necessary choke inductance in an amplifier or other RF system is dependent on five things:

RF voltage across the choke's impedance

Choke Q and ability to dissipate heat

Bypass capacitor's ability to handle current

Q increase that can be tolerated in the PA anode system

The extra plate tuning capacitance available

RF choke impedance varies widely with frequency. At low frequencies the choke
looks like an inductor either shunting (parallel equivalent) or in series
(series equivalent) with a resistance. Let's look at a Heathkit and Ameritron
choke I designed in the late 1980's.

This choke has the following characteristics (from an Excel spreadsheet I used
in AL80B design work):

F MHz

L or C

Xs

Rs

Xp

Rp

Q

1.8

255 µH

2550

22

2550

295590

116

3.5

275 µH

6060

85

6061

432127

71

7

.36 pF

-315000

3851

-327203

26764200

82

10.15

2.3 pf

-6900

79

-6901

602737

87

12.1

0

0

504

0

504

0

14

.6 pF

-19200

380

-19952

1008110

51

15.7

0

0

550

0

550

0

18.2

.6 pF

-14300

220

-14357

933205

65

21

1 pF

-7800

126

-7846

486452

62

24.8

1.25 pF

-5240

87

-5368

467250

60

28

1.37 pF

-4130

69

-4229

291096

60

Rp = Rs
+ Xs2
/ Rs

Xp = Xs
+ Rp2
/ Xs

Note: L, C, and some other values
will not be textbook perfect because they are subject to measurement
tolerance and rounding errors.
Values at or near parallel resonance (40 and 30 meters) may be subject to
impedance measurement errors because measurements of extreme impedances is
difficult.

Choke Heating and RF Current

Choke RF dissipation can be determined by voltage across the choke and Rp of
the choke. The standard formula E^2/R applies. The above choke on 160 meter
looks like:

Using E^2/Rp and assuming an RMS tank voltage of .6 times dc voltage, at 2800
volts dc supply we would have a maximum choke RF power dissipation of
(2800*.6)^2 / 295590 = 9.55 watts

The model to the left approximates these values, and is what we
would expect for a 1000 watt output class AB power amplifier with 2800 volts dc
on the anode.

We can see how critical choke Q becomes. We can reduce choke impedance if
choke Q is high, but we have to be mindful of low choke impedances with low Q.
In other words, choke Q becomes increasingly critical as choke impedance is
reduced. This, and saturation problems, are why magnetically-soft iron-core
chokes are generally a bad idea in high-impedance circuits of high-power
multiple band amplifiers.

Below are typical
choke dissipations based on an amplifier plate voltage of 3000 volts for 160-10
meters. Spreadsheet like this show why operation on or near series-resonant points is
catastrophic! Also, heat dissipation is not spread over the entire length of the
choke. On higher frequencies, and especially near series-resonance, heat (or
loss) is concentrated in certain areas of the winding. This is why calculations
or series-resonance measurements alone will not prove safe operation, although
they clearly will indicate unsafe operation.

Below is a copy of an excel spreadsheet showing worse case
dissipation (Pd) calculations used in my AL80B amplifier design: